Scientists examine links between Northland deluge and climate change

Citizen scientists have helped NIWA determine that climate change has likely increased the chances of extreme rainfall that deluged Northland last year.

Citizen scientists have helped NIWA determine that climate change has likely increased the chances of extreme rainfall that deluged Northland last year.

Over five days in early July 2014, Northland was hit by extremely high rainfall when a low pressure system carrying exceptionally moist air became parked over the region. The town of Kaikohe recorded a July rainfall total more than 300% of normal, and total insurance claims from the deluge topped $18 million.

Scientists have been investigating to what degree human interference in the climate system influenced the risk of such extreme rainfall occurring. The results have been included in a paper in the special international report ‘Explaining Extreme Events of 2014 from a Climate Perspective’, published today by the Bulletin of the American Meteorological Society.

NIWA climate scientist Dr Suzanne Rosier, the paper’s lead author, was able to use computing time from a network of volunteers, including New Zealanders, to process thousands of simulations of regional climate models.

The volunteers take part in the ‘weather @ home’ project (part of the Oxford-based ‘’), in which they donate their computers’ spare processing power to crunch weather data from climate models.

The computing power harnessed in this way enables far more information to be gathered than would be possible with conventional computing resources, helping to increase confidence in the scientific outcomes.

For the Northland study, Dr Rosier used the state-of-the-art Australia New Zealand regional climate model to run two sets of experiments. The first looked at the five-day weather data under present climate conditions, while the second analysed the weather data under climate conditions as they might have been had there been no human influence.

“The advantage in using the weather @ home model is that we can run thousands of simulations. Since weather patterns are chaotic, we need such a large number of simulations to help us estimate the range of possible weather outcomes under any particular set of driving conditions. By having this very large number of estimates both with and without the human influence on climate we are able to draw more confident conclusions about the human impact on extreme weather events than would otherwise be the case.”

While the rainfall that led to the 2014 Northland floods could have occurred in a world without climate change, Dr Rosier found that the chances of such heavy rain have increased as a result of human influence. The study’s best estimate is that the risk of such an event has approximately doubled because of human interference with the climate system. 

This follows similar studies by NIWA released in the past couple of years analysing the effects of climate change on two previous extreme events: extreme rainfall in Golden Bay and Nelson in 2011 that flooded more than 300 homes and properties, and severe drought across the North Island in early 2013.

The Golden Bay study concluded that an increase in greenhouse gases helped increase atmospheric moisture levels, contributing to extreme rainfall in the area. In the case of the 2013 drought, scientists found that human influence had made conditions more favourable for drought.

“Our contribution to this latest report continues NIWA’s focus on understanding how man-made changes to the climate system could be affecting today’s extreme weather events, and the influence climate change might be having on the severity and frequency of those events,” Dr Rosier said.

You can support the project by donating your spare computing power. Just sign up at:

Click here to see the full report